WARSAW/VIENNA, June 28 (Reuters) - Poland's Financial Supervision Authority (KNF) said on Wednesday it had approved the initial public offering (IPO) prospectus for the Polish unit of Austria's Raiffeisen Bank International (RBI) but the exact timing of the issue remained unclear.

RBI has pledged to list 15 percent of shares in Raiffeisen Bank Polska, also known as Raiffeisen Polbank IPO-RBP.WA, on the Warsaw bourse.

KNF had put a June 30 deadline on the IPO. But this month RBI said the timing would depend on market conditions. RBI Chief Johann Strobl told newspaper Kleine Zeitung this week the IPO would not take place at any cost.

Austrian news agency APA, without citing its source, reported on Wednesday that RBI had agreed with KNF on a clause for the IPO saying that it would not float the stake if this would cause an "unreasonable" loss.

APA cited an unnamed expert saying that a 10 percent discount to the book value of 220 million euros ($249.85 million) would be acceptable, but not a 50 percent discount.

RBI declined to comment on the APA report.

Earlier this year, RBI put the book value of its Polish unit at about 1.4 billion euros.

When asked about investor interest, a person familiar with the IPO discussions told Reuters: "You can sense the mood and it's not phenomenal."